In the design of machines for the manufacture of envelopes and the like, it is common practice to apply the glue, adhesive, or gum by an "offset" process in a manner similar to printing. The offset apparatus receives the glue from a roller with which it comes in contact and it then transfers the gum onto the envelope blank. It will be readily appreciated that, in order that the apparatus may operate properly, the gum must be spread very evenly on the transfer roller and without defects in the gum. The layer of gum has been applied in the past by using a primary roller whose periphery resides in a body of gum and which contacts the transfer roller while rotating in the same direction, so that the gum resides in a pool in the nip between the two rollers. The thickness of the layer of gum which is lifted from the gum box by the primary roller and carried over to the transfer roller is regulated by a doctor blade or roller whose operative edge is fixed in spaced relationship to the surface of the primary roller, so that only the desired amount of gum passes through this gap and the excess is scraped back into the gum box. There are a number of problems associated with this arrangement, one of them being that a change in viscosity of the gum can cause large changes in the thickness of the coat. Of course, too much gum is wasteful of the expensive gum. Too thin a coat results in an inoperative envelope and in customer complaints, if the defective envelope should pass through the inspector's observing eyes. The old arrangements also result in excessive churning of the gum, which means that the gum is mixed with oxygen and, therefore, oxidizes and thickens. More bubbles of hardened glue are passed on to the envelope as well as hardened particles that may accumulate on the blade before eventually being released and passed on to the transfer mechanism. Furthermore, the churning causes a hardening of the glue on the edge of the doctor blade that results in localized streaking and results in an envelope whose appearance is less than desirable and whose elements are not properly cemented together. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
A gum applicator capable of providing a transfer roller with an even layer of gum which is free of particles or streaks.
It is, therefore, an outstanding object of the invention to provide a gum applicator for use with an envelope machine, in which the gum is subjected to very little mechanical agitation.
Another object of this invention is the provision of a gum box in which circulation of gum takes place from the ends toward the middle and back again to produce an even distribution of gum across a primary roller and to keep that gum thoroughly mixed and at a constant viscosity.
A further object of the present invention is the provision of a gum box which can be retro-fitted to existing envelope-making machines without substantial alteration of the machine.
It is another object of the instant invention to provide a gum box in which the operative elements can be readily cleaned without complete disassembly thereof or removel from the envelope machine.
A still further object of the invention is the provision of a gum box which is simple in construction, which is inexpensive to manufacture, and which is capable of a long life of useful service with a minimum of maintenance.
A further object of this invention is to provide a small secondary reservoir with a fresh gum supply from which the transfer roll (which runs at the relatively high machine surface speeds) can draw its needs with a minimum of agitation to the entire adhesive supply.